The mistakes that cost the most
- Leaving it until late January. Systems are slow, questions go unanswered, and the £100 penalty lands the moment you miss 31 January, even if no tax is due. Register and start early.
- Forgetting a source of income. A second client, savings interest, a few dividends, a bit of rent. HMRC cross checks against banks and employers, so gaps are noticed. List every income source before you begin.
- Guessing expenses. Round numbers and missing receipts invite questions. Claim only what you can evidence, and keep the records for at least 5 years after the deadline.
- Missing reliefs. Higher rate taxpayers who pay into a personal pension often fail to claim the extra relief, and simply overpay. Gift Aid works the same way.
- Ignoring payments on account. If your bill tops £1,000, HMRC asks for advance instalments toward next year in January and July. Not expecting it turns January into a shock.
- Mixing up the tax year. The 2025/26 return covers 6 April 2025 to 5 April 2026. Entering figures from the wrong year throws the whole return off.
- Not claiming the trading or property allowance. If your income from a side activity is under £1,000, the allowance can remove the need to pay tax on it at all.
- Forgetting the High Income Child Benefit Charge. If you or your partner earns above the threshold and claim Child Benefit, it must be reported here.
- Paying to the wrong reference. Use your UTR followed by the letter K for Self Assessment, or the payment may not be matched to your account.
- Not amending a known error. You can correct an online return up to 12 months after the deadline. Fixing it early is always better than waiting for HMRC.
- Assuming no tax means no return. If HMRC has asked you to file, you must, even if the result is zero, or the penalty still applies.
One missed box can cost more than a fixed fee
Most of these mistakes are invisible until HMRC writes to you. If your return has self employment, property, dividends or a first year of trading, having an accountant check it is usually cheaper than the penalty or the overpaid tax. TaxTune prepares, checks and files the whole thing for you.
Frequently asked questions
What is the most common Self Assessment mistake?
Filing late is the most common and the most avoidable. The online deadline is 31 January and the £100 penalty applies immediately, even when no tax is owed. Starting in the autumn removes almost all of the risk.
What happens if I forget some income?
HMRC receives data from banks, employers and other sources, so undeclared income is often spotted. You may face additional tax, interest and a penalty. If you realise after filing, amend the return promptly to reduce any penalty.
How long should I keep my records?
Keep Self Assessment records for at least 22 months after the end of the tax year if you are not in business, and at least 5 years after the 31 January deadline if you are self employed.
Can I fix a mistake after I have filed?
Yes. You can amend an online return up to 12 months after the filing deadline by signing back in. After that you must write to HMRC. Correcting early always looks better than a late discovery.
Do I still file if I owe nothing?
If HMRC has issued you a notice to file, you must submit the return even if the tax due is zero. Otherwise the automatic late filing penalty still applies.
Never worry about a Self Assessment mistake again
Send us your figures and we will prepare the return, claim every relief you are entitled to, check it line by line, show you the bill before filing, and submit it to HMRC on time. Fixed fee, agreed up front, no January panic.